This invention relates to loudspeakers systems.
In a conventional speaker system the mounting is usually a cabinet at the rear of the loudspeaker drive unit into which sound passes. The main function of the rear cabinet is to prevent the rearward radiation coming round to the front and then, since it is out of phase, cancelling out the frontward radiation.
The rear box can be totally enclosed, in which case all the direct rearward sound is prevented from coming round to the front. Or the box may have a hole in it, in which case the mass of air in the hole and the stiffness of the air in the box produce a “Helmholtz” resonance which may be used to reinforce the bass response of the speaker.
Other ways of preventing the rear-front cancellation are (a) to place the speaker on a large baffle and (b) to pass the sound down a tube of sufficient length so that when it emerges from the end, at the later time which depends on the length, it is in phase with the low frequency part of the front radiation which it then reinforces rather than cancels.
Furthermore, the use of tubes as damped quarter-wave (or longer) transmission lines for absorbing the unwanted acoustic output from the rear of loudspeaker is now well established. It is also known that tapering the cross-sectional area of such a device in the manner of an inverse horn significantly improves the absorption efficiency. The tapering tube is acoustically coupled to the rear of a loudspeaker drive unit to lead away and absorb sound waves produced at the rear of the loudspeaker drive unit.
GB-A-2 290 672 discloses a loudspeaker system comprising a bass unit, a mid-range unit, a treble unit, and a tweeter unit. Each of the units includes a respective loudspeaker drive unit. The mounting for the loudspeaker drive unit is such that there is substantially no rear reflecting surface behind the diaphragm of the loudspeaker drive unit. The pole piece of the respective magnet system of each loudspeaker drive unit is provided with an aperture through which, in use, sound from the rearward side of the diaphragm passes. Each of the loudspeaker drive units has a respective circular-section tube extending from the rear of the loudspeaker drive unit. Each tube contains sound-absorbent material such as glass fibre and tapers away from the associated loudspeaker drive unit.
Patent specification WO 98/51121 discloses a loudspeaker system which comprises: a loudspeaker drive unit and a tube acoustically coupled to the rear of the loudspeaker drive unit for leading away and absorbing sound waves produced at the rear of the loudspeaker drive unit. The tube is acoustically coupled to the loudspeaker drive unit by means of a hollow resonant enclosure and the loudspeaker drive unit is mounted at an aperture in an external wall of the enclosure. The tube communicates with the interior of the enclosure and extends outwardly from the enclosure.
A disadvantage of the known systems having a tube at the rear of a loudspeaker drive unit is that they are, at least in the case of bass and mid-range units, cumbersome and are not therefore very satisfactory for home use where space is often at a premium.
In recent years, so-called flat panel loudspeaker systems (the term “loudspeaker system” being used to mean the combination of at least one loudspeaker drive unit and a loudspeaker enclosure) have been introduced of which the overall depth is much reduced in comparison with a loudspeaker system of traditional design. The reduced depth is possible because mid-range and bass loudspeaker drive units with a reduced front to back dimension have been developed.
The known systems having a tube at the rear of a loudspeaker drive unit are virtually unusable in a flat panel loudspeaker system on the ground of spatial restrictions.